A rolling resistance of a tire is one of important measurement items for measuring characteristics and performances of tires for a truck, an automobile, and the other vehicles. The rolling resistance of the tire is a force acting between the tire and the ground surface in the tangential direction, and is measured as a force Fx (a change in the rolling resistance Fx when a pressing load Fz is changed) acting between a test tire and a simulation road surface of a drum or the like in the tangential direction in a rolling resistance testing machine.
A typical method of measuring the rolling resistance Fx is a method using a drum type rolling resistance testing machine. The drum type rolling resistance testing machine is configured to measure a relation between the pressing load Fz and the rolling resistance Fx by a multi component force detector (load cell) provided in a spindle shaft supporting a tire while the tire is pressed against a simulation road surface formed in the outer periphery of a running drum.
Specifically, in a case where the rolling resistance Fx is measured, the load fx in the rolling resistance direction is measured by the multi component force detector provided in the spindle shaft, and Fx may be calculated by “Fx=fx(L/Rd)” (a load method). Here, Rd indicates the radius of the running drum, and L indicates the distance between the axes of the running drum and the tire spindle shaft. Further, as another method, there is known a method in which a driving torque τfor rotating the running drum is measured and the rolling resistance Fx is measured by “Fx=τ/Rd” (a torque method).
Incidentally, in such a rolling resistance testing machine, there is a need to calibrate the multi component force detector for using the testing machine. In addition, when the multi component force detector is used for a long period of time, a detection value may have an error. Thus, for example, the multi component force detector needs to be calibrated every predetermined usage period.
Various methods of calibrating the multi component force detector have been developed. However, as illustrated in Patent Document 1, a calibration may be performed after adding a test load in respective directions using a weight of a given mass. Further, as illustrated in Patent Document 2 or Patent Document 3, there is disclosed a method of performing a calibration by applying an external force through a highly precise load detector.